Writing Testbenches

Accessing the design

When cocotb initializes it finds the toplevel instantiation in the simulator and creates a handle called dut. Toplevel signals can be accessed using the “dot” notation used for accessing object attributes in Python. The same mechanism can be used to access signals inside the design.

# Get a reference to the "clk" signal on the toplevel
clk = dut.clk

# Get a reference to a register "count"
# in a sub-block "inst_sub_block"
count = dut.inst_sub_block.count

Assigning values to signals

Values can be assigned to signals using either the value property of a handle object or using direct assignment while traversing the hierarchy.

# Get a reference to the "clk" signal and assign a value
clk = dut.clk
clk.value = 1

# Direct assignment through the hierarchy
dut.input_signal <= 12

# Assign a value to a memory deep in the hierarchy
dut.sub_block.memory.array[4] <= 2

The syntax sig <= new_value is a short form of sig.value = new_value. It not only resembles HDL syntax, but also has the same semantics: writes are not applied immediately, but delayed until the next write cycle. Use sig.setimmediatevalue(new_val) to set a new value immediately (see setimmediatevalue()).

Signed and unsigned values

Both signed and unsigned values can be assigned to signals using a Python int. Cocotb makes no assumptions regarding the signedness of the signal. It only considers the width of the signal, so it will allow values in the range from the minimum negative value for a signed number up to the maximum positive value for an unsigned number: -2**(Nbits - 1) <= value <= 2**Nbits - 1 Note: assigning out-of-range values will raise an OverflowError.

A BinaryValue object can be used instead of a Python int to assign a value to signals with more fine-grained control (e.g. signed values only).

module my_module (
    input   logic       clk,
    input   logic       rst,
    input   logic [2:0] data_in,
    output  logic [2:0] data_out
# assignment of negative value
dut.data_in <= -4

# assignment of positive value
dut.data_in <= 7

# assignment of out-of-range values
dut.data_in <= 8   # raises OverflowError
dut.data_in <= -5  # raises OverflowError

Reading values from signals

Values in the DUT can be accessed with the value property of a handle object. A common mistake is forgetting the .value which just gives you a reference to a handle (useful for defining an alias name), not the value.

The Python type of a value depends on the handle’s HDL type:

  • Arrays of logic and subtypes of that (sfixed, unsigned, etc.) are of type BinaryValue.

  • Integer nets and constants (integer, natural, etc.) return int.

  • Floating point nets and constants (real) return float.

  • Boolean nets and constants (boolean) return bool.

  • String nets and constants (string) return bytes.

For a BinaryValue object, any unresolved bits are preserved and can be accessed using the binstr attribute, or a resolved integer value can be accessed using the integer attribute.

>>> # Read a value back from the DUT
>>> count = dut.counter.value
>>> print(count.binstr)
>>> # Resolve the value to an integer (X or Z treated as 0)
>>> print(count.integer)
>>> # Show number of bits in a value
>>> print(count.n_bits)

We can also cast the signal handle directly to an integer:

>>> print(int(dut.counter))

Concurrent and sequential execution

An await will run an async coroutine and wait for it to complete. The called coroutine “blocks” the execution of the current coroutine. Wrapping the call in fork() runs the coroutine concurrently, allowing the current coroutine to continue executing. At any time you can await the result of the forked coroutine, which will block until the forked coroutine finishes.

The following example shows these in action:

# A coroutine
async def reset_dut(reset_n, duration_ns):
    reset_n <= 0
    await Timer(duration_ns, units="ns")
    reset_n <= 1
    reset_n._log.debug("Reset complete")

async def parallel_example(dut):
    reset_n = dut.reset

    # Execution will block until reset_dut has completed
    await reset_dut(reset_n, 500)
    dut._log.debug("After reset")

    # Run reset_dut concurrently
    reset_thread = cocotb.fork(reset_dut(reset_n, duration_ns=500))

    # This timer will complete before the timer in the concurrently executing "reset_thread"
    await Timer(250, units="ns")
    dut._log.debug("During reset (reset_n = %s)" % reset_n.value)

    # Wait for the other thread to complete
    await reset_thread
    dut._log.debug("After reset")

See Coroutines for more examples of what can be done with coroutines.

Forcing and freezing signals

In addition to regular value assignments (deposits), signals can be forced to a predetermined value or frozen at their current value. To achieve this, the various actions described in Assignment Methods can be used.

# Deposit action
dut.my_signal <= 12
dut.my_signal <= Deposit(12)  # equivalent syntax

# Force action
dut.my_signal <= Force(12)    # my_signal stays 12 until released

# Release action
dut.my_signal <= Release()    # Reverts any force/freeze assignments

# Freeze action
dut.my_signal <= Freeze()     # my_signal stays at current value until released

Accessing Identifiers Starting with an Underscore

The attribute syntax of dut._some_signal cannot be used to access an identifier that starts with an underscore (_, as is valid in Verilog) because we reserve such names for cocotb-internals, thus the access will raise an AttributeError.

A workaround is to use indirect access using _id() like in the following example: dut._id("_some_signal", extended=False).

Passing and Failing Tests

A cocotb test is considered to have failed if the test coroutine, or any coroutine fork()ed by the test coroutine, fails an assert statement or raises a cocotb.result.TestFailure. Below are examples of failing tests.

async def test(dut):
    assert 1 > 2, "Testing the obvious"

async def test(dut):
    raise TestFailure("Reason")

async def test(dut):
    async def fails_test():
        assert 1 > 2
    await Timer(10, 'ns')

async def test(dut):
    async def fails_test():
        raise TestFailure("Reason")
    await Timer(10, 'ns')

When a test fails, a stacktrace is printed. If pytest is installed and assert statements are used, a more informative stacktrace is printed which includes the values that caused the assert to fail. For example, see the output for the first test from above.

0.00ns ERROR    Test Failed: test (result was AssertionError)
                Traceback (most recent call last):
                  File "test.py", line 3, in test
                    assert 1 > 2, "Testing the obvious"
                AssertionError: Testing the obvious

A cocotb test is considered to have errored if the test coroutine, or any coroutine fork()ed by the test coroutine, raises an exception that isn’t considered a failure. Below are examples of erroring tests.

async def test(dut):
    await coro_that_does_not_exist()  # NameError

async def test(dut):
    async def coro_with_an_error():
        dut.signal_that_does_not_exist <= 1  # AttributeError
    await Timer(10, 'ns')

When a test ends with an error, a stacktrace is printed. For example, see the below output for the first test from above.

0.00ns ERROR    Test Failed: test (result was NameError)
                Traceback (most recent call last):
                  File "test.py", line 3, in test
                    await coro_that_does_not_exist()  # NameError
                NameError: name 'coro_that_does_not_exist' is not defined

If a test coroutine completes without failing or erroring, or if the test coroutine, or any coroutine fork()ed by the test coroutine, raises cocotb.result.TestSuccess, the test is considered to have passed. Below are examples of passing tests.

async def test(dut):
    assert 2 > 1  # assertion is correct, then the coroutine ends

async def test(dut):
    raise TestSuccess("Reason")  # ends test with success early
    assert 1 > 2  # this would fail, but it isn't run because the test was ended early

async def test(dut):
    async def ends_test_with_pass():
        raise TestSuccess("Reason")
    await Timer(10, 'ns')

A passing test will print the following output.

0.00ns INFO     Test Passed: test


Cocotb uses the builtin logging library, with some configuration described in Logging to provide some sensible defaults. Any forked coroutine holds a logging.Logger, and can be set to its own logging level.

task = cocotb.fork(coro)
task.log.debug("Running Task!")

The DUT and each hierarchical object can also have individual logging levels set. When logging HDL objects, beware that _log is the preferred way to use logging. This helps minimize the change of name collisions with an HDL log component with the Python logging functionality.

dut.my_signal._log.info("Setting signal")
dut.my_signal <= 1